1. Field of Invention
This invention relates to induced current linear and rotary position transducers.
2. Description of Related Art
Various induced current position transducers are known. U.S. Pat. No. 5,973,494, incorporated herein by reference in its entirety, discloses an electronic caliper using an induced current position transducer. U.S. Pat. No. 6,005,387, incorporated herein by reference in its entirety, discloses various reduced-offset high accuracy induced current position transducers and associated signal processing techniques. U.S. Pat. No. 6,329,813 and U.S. patent application Ser. No. 09/421,497, each incorporated herein by reference in its entirety, disclose various reduced-offset high accuracy induced current absolute position transducers and associated signal processing techniques. U.S. Pat. Nos. RE037490 and 6,049,204, each incorporated herein by reference in its entirety, disclose reduced-offset high accuracy induced current calipers and reduced-offset induced current linear scales respectively, as well as associated signal processing techniques.
A reduced-offset-type induced current position transducer generally includes a transmitter winding, a corresponding receiver winding and a signal generating and processing circuit on a readhead. Reduced-offset-type induced current position transducers are configured so that the transmitter winding and the corresponding receiver winding occupy separate regions on the readhead. The transmitter winding and the corresponding receiver winding are spaced apart in a direction transverse to the measuring axis of the position transducer. The transducer also includes a scale having at least one scale loop. The transmitter winding is inductively coupled to a first portion of the scale loop and a second portion of the scale loop is, in turn, inductively coupled to the corresponding receiver winding.
When a time-varying signal originating from the signal generating and processing circuit passes through the transmitter winding, a primary magnetic field is generated. The transmitter winding is inductively coupled to the first portions of the scale loops by the primary magnetic field. The second portions of the scale loops generate secondary magnetic fields. The receiver winding is inductively coupled to the second portions of the scale loops by the secondary magnetic fields.
At least one of the transmitter winding or the receiver winding is formed in a periodic pattern, such as a sinusoidal pattern, having dimensions corresponding to the coupling loops. Various winding configurations are known in the art to reduce or cancel the extraneous inductive coupling in the device. The receiver windings inductively couple with the second loop portions of the scale loops to differing degrees, depending on the position of the scale relative to the readhead.
U.S. Pat. No. 6,011,389, incorporated herein by reference in its entirety, discloses an incremental induced current position transducer. U.S. Pat. Nos. 5,804,963, 4,853,684 and 6,259,249, each incorporated herein by reference in its entirety, disclose various other types of induced current position transducers. The incorporated ""389, ""963, ""684 and ""249 patents include scale configurations wherein periodically arranged scale elements, in some embodiments, comprise conductive loops.
U.S. patent application Ser. No. 09/987,400, also incorporated herein by reference in its entirety discloses induced current position transducers having improved scale loop structures.
While each of the above-described references provides commercially viable induced current position transducers, there is always need for devices that can accomplish the functions of known devices at less expense. To this end, this invention provides reduced-offset-type induced current position transducers that perform with necessary signal gain and acceptable levels of position error, but that can be fabricated and assembled at a reduced cost.
The purpose of a scale in an induced current position transducer is to carry a pattern of induced currents that give rise to a periodically varying magnetic field. The magnetic field and/or the induced currents can be sensed by the receiver windings. This is accomplished by inducing a current with a transmitter winding held close to the scale, and manipulating conditions so that the current will flow as desired in the scale region where the periodic magnetic field is desired in order to determine the scale position. Scales used in known reduced-offset-type induced current position transducers are typically configurations of electrically isolated conductive loops. Such electrically isolated conductive loops can be manufactured by printing a conductive trace on a circuit board. A current is induced in the part of the loop that is close to the transmitter, and the induced current flows around the loop to determine the field measured by the receivers.
The reduced-offset-type induced current position transducers according to this invention can include conductive tape scales. The tape scales serve the same purpose as known scales, in the sense that they carry a pattern of induced currents that give rise to a periodically varying magnetic field, but they do so without the use of electrically isolated loops in which current is confined. The tape scales according to this invention accomplish this task through the use of apertures in the tape scale itself. When current is induced in a tape scale, the current flows along the surface of the tape scale. The induced current is present throughout the conductive sheet forming the tape scale, but is concentrated where the conductive sheet is closest to a transmitter winding. When apertures are added to the tape scale, as in the tape scales according to this invention, the induced current must flow around the apertures. Thus, the position of the apertures determines the pattern of the current induced by the transmitter. By carefully configuring the placement and size of apertures in the tape scale, the location and the direction of the current can be predicted and controlled, and accurate measurements can be obtained when the tape scale is employed in a reduced-offset-type induced current position transducer.
In various exemplary embodiments, the reduced-offset-type induced current position transducer according to this invention includes a first member and a second member having a measuring axis. The first member is movable along the measuring axis. At least one magnetic field generator is positioned on one of the first and second members. Each magnetic field generator generates a first changing magnetic flux in a respective first region in response to a drive signal.
In various exemplary embodiments, the other of the first and second members comprises a plurality of apertures formed in a conductive strip so that a first portion of each aperture is positionable within a respective first region and a second portion of each aperture is positionable in a respective second region distinct from the respective first region. An induced current is generated in the conductive strip in response to the first changing magnetic flux, the induced current including induced current flow in the respective second region. The apertures determine the pattern of induced current flow and the associated periodically varying magnetic field in the respective second region.
In various exemplary embodiments, at least one magnetic flux sensor is positioned on one of the first and second members. At least one of the at least one magnetic flux sensor and the at least one magnetic field generator includes a conductive trace or wire that extends along the measuring axis, the placement of the wire-like conductor transverse to the measuring axis being spatially modulated along the measuring axis so that the direction of the conductor is alternatingly in a first direction transverse to the measuring axis and a second direction, which is opposite to the first direction, transverse to the measuring axis. Each magnetic flux sensor is positioned outside a respective first flux region in a respective second region and is sensitive to at least one of the pattern of induced current flow and the associated periodically varying magnetic field in the respective second region. The inductive coupling between the alternating structure of the magnetic flux sensor and the pattern of induced currents and the associated periodically varying magnetic field in the respective second region varies as the magnetic flux sensor moves relative to the conductive strip and apertures along the measuring axis. Each magnetic flux sensor generates an output signal which is a function of the relative position between the magnetic flux sensor and at least some of the apertures.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.